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Chemistry LibreTexts

1: Atoms

  • Page ID
    25441
    • 1.1: A Particulate View of the World - Structure Determines Properties
      The chemistry-centered reformulation of this principle is that Structure Determines Properties, which is a useful concept in chemistry and in fields that chemistry is important including biology, environmental science, biochemistry, polymer science, medicine, engineering, and nutrition among many others.
    • 1.2: Classifying Matter- A Particulate View
      Under normal conditions, there are three distinct states of matter: solids, liquids, and gases. Solids are relatively rigid and have fixed shapes and volumes. In contrast, liquids have fixed volumes but flow to assume the shape of their containers, such as a beverage in a can. Gases, such as air in an automobile tire, have neither fixed shapes nor fixed volumes and expand to completely fill their containers.
    • 1.3: The Scientific Approach to Knowledge
      The particulate model of matter is not a clear conclusion to the casual observer of nature. While all modern scientists accept the concept of the atom, when the concept of the atom was first proposed about 2,500 years ago, ancient philosophers laughed at the idea. It has always been difficult to convince people of the existence of things that are too small to see. We will spend some time considering the evidence (observations) that convince scientists of the existence of atoms.
    • 1.4: Early Ideas about the Building Blocks of Matter
      Greek philosophers were "all thought and no action" and did not feel the need to test their theories with reality via experiments and the scientific method. Dalton's atomic theory were based on experimentation and testing ideas against reality.
    • 1.5: Modern Atomic Theory and the Laws That Led to It
      With the development of more precise ideas on elements, compounds and mixtures, scientists began to investigate how and why substances react. French chemist A. Lavoisier laid the foundation to the scientific investigation of matter by describing that substances react by following certain laws. These laws are called the laws of chemical combination. While John Dalton is credited for proposing modern atomic theory. Dalton built his atomic theory upon these laws.
    • 1.6: The Discovery of the Electron
      The atom was not indivisible as Dalton's theory had postulated. J. J. Thomson proved that atoms were not the most basic form of matter. He demonstrated that cathode rays consist of charged particles, later identified as electrons. He measures the ratio of charge over mass of an electron, Robert Millikan carried out a series of experiments using electrically charged oil droplets, which allowed him to calculate the charge on a single electron.
    • 1.7: The Structure of The Atom
      The results of Thomson’s and other experiments implied that electrons were constituents of all matter and hence of all atoms. Since macroscopic samples of the elements are found to be electrically neutral, this meant that each atom probably contained a positively charged portion to balance the negative charge of its electrons.
    • 1.8: Subatomic Particles - Protons, Neutrons, and Electrons
      To date, about 118 different elements have been discovered; by definition, each is chemically unique. To understand why they are unique, you need to understand the structure of the atom (the fundamental, individual particle of an element) and the characteristics of its components. Atoms consist of electrons, protons, and neutrons. Although this is an oversimplification that ignores the other subatomic particles that have been discovered, it is sufficient for discussion of chemical principles.
    • 1.9: Atomic Mass- The Average Mass of an Element’s Atoms
      There are 21 elements with only one isotope, so all their atoms have identical masses. All other elements have two or more isotopes, so their atoms have at least two different masses. However, all elements obey the law of definite proportions when they combine with other elements, so they behave as if they had just one kind of atom with a definite mass. To solve this dilemma, we define the atomic mass as the weighted average mass of all naturally occurring isotopes of the element.
    • 1.10: The Origins of Atoms and Elements
      The Earth is composed of 90 chemical elements, of which 81 have at least one stable isotope. Most of these elements have also been detected in stars. Where did these elements come from? The accepted scenario is that the first major element to condense out of the primordial soup was helium, which still comprises about one-quarter of the mass of the known universe. Stellar nucleosynthesis then is responsible for the generation of new elements by nuclear reaction within stars.

    Thumbnail: Schematic of Rutherford's famous gold foil experiment.